In response to ever-increasing user needs, self-propelled cranes have been made capable of lifting ever greater loads. While a number of factors enter into determining crane capacity, a basic limitation arises from the fact that, inevitably, the weight of the crane and its load must be transferred to the earth in some stable fashion and, if rotation of the load is desired, the crane/earth connection must be made stable throughout the arc of crane rotation.
A significant increase in crane capacity was achieved by providing a self-propelled crane with the support ring and extended boom carrier disclosed and claimed in U.S. Pat. Nos. 3,485,383 and 3,878,944. In these designs, the weight of the crane and its load is transferred to the ground through a large diameter, track-like ring. As shown in these patents, and as practiced commercially for some years, the support ring is either blocked into place by timbers fitted and wedged beneath and completely around the ring or is supported by a plurality of jacks spaced around the periphery of the ring.
Further refinements in ring supported cranes are disclosed in U.S. Pat. Nos. 4,042,115 and 4,103,783. These patents disclose, inter alia, that a separate transporter mechanism may be run in and out of an otherwise stationary ring-supported crane in order to move that crane between different locations. Alternatively, transporter mechanisms and/or idler crawlers or dollies may be installed beneath the ring under the boom foot and counterweight.
The stability of the earth-crane connection was significantly improved by the crane disclosed in copending parent application Ser. No. 128,139, now U.S. Pat. No. 4,358,021. This application discloses a lift crane having its counterweight mounted on a rear, ring-supported rotatable deck. A front ring segment supports a boom carrier on which a forwardly inclined boom and a rearwardly inclined mast are pivotally mounted. The tip of the mast, the tip of the boom and the counterweight are interconnected by rigging with a swivel connection between the mast tip and the counterweight. The carrier is mounted on rollers for movement on the front ring segment and, as the carrier pivots about a first vertical axis intermediate of the boom carrier and the rear ring, the counterweight pivots about a second vertical axis running through the center of the rear ring. Due to this crane's ability to move a load through a limited arc without having to activate its transporter assemblies, it has been found to be particularly useful in accurately placing a heavy load on mounting blocks, bolts or the like.
While the dual pivot crane of copending application Ser. No. 128,139, now U.S. Pat. No. 4,358,021 constitutes a significant improvement in traveling lift cranes, it was determined that the load capacity of this type of crane is limited by the fact that side loads are induced in the rearwardly inclined mast when the crane is pivoted due to the tip of the mast and the point on the rear deck to which the mast tip is connected by rigging moving out of alignment when viewed from above. (When aligned, the mast is subjected to substantially no side loads.) Thus, the primary object of the present invention is to provide a heavy lift crane assembly having a separated front ring segment with a boom and mast mounted on a boom carrier for rotation on the ring segment spaced outwardly from the rear works in which the torsional and side loads induced in the mast due to rotation of the carrier are minimized.